Calcareous Pebbles formed by Algce. 75 



Gomont in the mass I sent him. At the same time he says, ' The interior 

 of the calcareous mass is formed of entangled filaments ; they appear to 

 belong to a Schisotkrix t \xo!i which?' It was precisely these sheaths that 

 had puzzled me, and I had been inclined to regard them as those of an 

 unknown ScJiizotJirix. They do not agree very closely with the figures 

 given of 5. fasciculata, but M. Gomont finally says, ' I do not think 

 one can make anything very distinctly out of this specimen, except 

 S. fasciculata, which undoubtedly occurs in abundance.' I have figured 

 the specimen selected by M. Gomont and others on plate XIX., figs. 5a 

 and b, and have given an indication (fig. 4) of the density with which the 

 filaments are interwoven. 



S. fascicidata is known from the countries of central Europe and forms 

 small, stony cushion-like calcareous masses. The filaments are always en- 

 tangled where they are incrusted, but on the surface, where they are more 

 or less free, there is very little entanglement and the filaments are almost 

 straight and parallel. The ordinary incrustation formed by this species is 

 said to be zoned in section, and its surface is generally mammillate. It 

 is constantly found growing in association with other Algae of the same 

 group, and there is a considerable colony of them in the present instance. 

 The most striking feature, however, in this respect is the large number 

 and variety of diatoms present in the mass. Mr. E. Grove has made an 

 exhaustive examination of them, and has most kindly furnished me with 

 the appended list of them, which extends to about 90 species and varieties. 



A matter of particular interest is the massive occurrence of car- 

 bonate of lime in association with a kind of organism that is commonly 

 so incrusted only in hot springs, and to some extent it raises the question 

 of the influence of temperature on the deposition of carbonate of lime. 

 Murray and Irvine have made a series of most interesting experiments 

 leading to the conclusion stated by Murray (Challenger Reports 

 Summary of Results, p. 1456) as follows : 



' The very slight development of carbonate of lime shells and other 

 carbonate of lime structures in the cold waters of the polar regions are 

 instructive when recalled in connexion with the massive coral reefs 

 constructed in the polar regions in Palaeozoic and even later geological 

 times. The waters of the ancient oceans must have had a temperature of 

 65 or 70 F. at the poles, for it has been shown that the deposition of 

 carbonate of lime is due to the secretion of carbonate of ammonia, one of 

 the effete or waste products of marine animals, which, decomposing the 

 soluble sulphate of lime in sea-water, produces insoluble carbonate of lime 

 to form shells, its precipitation taking place with great difficulty and very 

 slowly in cold water, but easily and very rapidly within the organism in 



